1. Field of the Invention.
This invention relates to dock loading equipment and in particular to dock levelers which are used to bridge the gap between a dock floor and a vehicle parked at the dock. It specifically relates to a counterbalance mechanism for a mechanical dock leveler.
2. Prior Art.
Mechanical dock levelers are typically mounted in a pit at a loading dock and use a frame as a supporting structure. The leveler has a deck pivotally mounted to the frame at one end a hinged lip mounted to the deck at the other end. The lip stores in a pendent position and is raised to rest on the bed of the vehicle thus bridging the gap that exists.
Mechanical dock levelers are typically upward biased, meaning that when the leveler is released the deck rises and the lip, via a mechanism, extends. The deck is then "walked down" until the lip rests on the truck bed. Generally a hold down mechanism is used to retain the deck in any position within the operative range of the leveler and when released the deck elevates. These devices employ counterbalance mechanisms, typically with extension springs.
U.S. Pat. No. 3,117,332 describes such a counterbalance mechanism with extension springs coupled to the deck at one end while another end of the springs is coupled to an arm that pivots relative to the frame. The arm carries a roller that rides on a cam surface to provide the necessary counterbalance force. This complex arrangement requires a central frame structure to support the roller arm pivot which adds cost and limits access for cleaning debris from the pit floor. Also, the spring adjustment bolt is at the back of the leveler which makes adjustment difficult. Additionally considerations of wear and maintenance exist for such arrangements.
A variation is the use of compression springs to provide the necessary counterbalancing force. U.S. Pat. No. 3,528,118 illustrates such an arrangement. A compression spring is mounted to the frame at one end and to a crank arm in turn connected to the deck. The spring thus acts through a crank arm reducing the efficiency of the device. The tube which guides the spring adds cost, causes frictional losses, and limits access for cleaning debris from the pit floor.
U.S. Pat. No. 4,525,887 describes the use of extension springs in combination with a linkage pivotally coupled to the frame and the deck. The links are pivotally connected to each other and the extension spring is connected to one of the links and the frame. The spring, acting at full efficiency may be smaller than those used in a compression mode. The linkage is designed so that the counterbalance force is slightly greater than the force of the deck thereby allowing the use of a smaller spring. This device however requires additional pivot points, increased friction in operation and higher manufacturing costs.